Relays and contactors are known devices used for switching of intended circuits/loads and the like. A relay is an electrically operated switch. Many known relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low power signal or where several circuits must be controlled by one signal. A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings.
In general, a simple electromagnetic relay consists of a coil assembly, a movable armature, and one or more sets of contacts, i.e. single throw system, double throw system, etc. The sets of contact include movable contacts, fixed normally open contacts, and fixed normally closed contacts. The armature is mechanically linked to one or more sets of moving contacts and is held in place by a spring.
When an electric current is passed through the coil assembly it generates a magnetic field that attracts the armature. The consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact(s). If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by the spring force, of the return spring toward its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays and contactors are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high voltage or current application it reduces arcing. In order to allow the proper movement of the contacts, the spring force is designed to be less than the force generated by the coil.
In the case of double throw contacts, the system dynamic forces are much more complex than with single throw contacts. The main difficulty lies in maintaining the contact pressure at the contact terminals of the normally closed points by use of the return spring. Thus a bulkier, more robust mechanism is required to achieve the force required to overcome the return spring on contact transfer. This often warrants the use of a larger coil which increases the size and cost of the switch, relay, or contactor.
Referring to FIG. 1, a system according the prior art is shown. The system has a set of normally closed fixed contacts P1 which forms the top of the assembly and it is the terminal side where the contactor offers the continuity, in the de-energized or rest position. A set of normally open fixed contacts P4 is provided on a base P5 and is activated when the coil is in the energized condition. A moveable contact set P3 is positioned between the fixed contacts P1, P4 and is moveable between them. A contact spring P2 cooperates with the movable contact set P3 to move the contact set with a pre-defined pressure. A core rod or armature P6 cooperates with a plunger P7 and carries the set of moveable contacts for actuation/transfer. A return spring P8 cooperates with the movable contacts P3. A magnetic coil assembly P10 having a coil lid P11, an inner coil P12, and coil shell P13 cooperates to move the movable contact set P3.
In operation, energizing the coil assembly P10 with a pre-designed voltage sets the flux around the system and causes plunger P7 to move down, thereby resulting in a downward movement of the core rod P6, which results in the compression of return spring P8. This results in transfer of position of moveable contacts P3 from being in contact with the normally closed fixed contacts P1 to being in contact with the normally open fixed contacts P4. De-energizing coil assembly P10 resets return spring P8, plunger P7, rod P6 and movable contacts P3 to their initial positions. Accordingly, the resultant force of the contact springs P8 alone determines the contact pressure on the normally closed fixed contacts P1. Due to the constrained parameters of this design, the return spring P8 has to be designed with a weaker pre-load (de-energized) for proper pickup and dropout, resulting in de-rating of the normally fixed contacts P1. Therefore, for an identical contact rating of current, a lower contact force at the normally closed fixed terminals results in higher voltage drop values than experienced at the normally open fixed terminals. This necessitates the side of the higher voltage drop be de-rated to a lesser amperage, in order to maintain acceptable voltage drop values and temperature rise limits which may otherwise ruin the system due to the higher drop values.
It would, therefore, be beneficial to provide a contact system which eliminates the problems associated with the prior art and which provides for bi-directional switching without any loss in the voltage drop values.